This invention relates to a coder interpolator circuit which outputs movable element position data by using an A-phase signal and a B-phase signal.
An absolute position encoder is available which is capable of detecting, as absolute position, the rotational position of a rotary shaft of a motor or the like for driving a machine tool, etc.
FIG. 3 is a block diagram of the interpolator circuit of a prior art absolute position encoder. An A-phase/B-phase signal generating unit 1 is formed by a rotary coding disk, a stationary coding disk, a light-emitting element, a light-receiving element and a signal output circuit, none of which are shown. The signal generating unit outputs sinusoidal and cosinusoidal A-phase and B-phase signals having predetermined cycles per revolution. AD converters 2 and 3 subject the A- and B-phase signals, respectively, to an analog -to- digital conversion (AD conversion) to obtain address information in the form of a set of the AD converter output data. Angle data (position data) D.sub.A conforming to the A- and B-phase signal levels is read out of the storage area of a ROM 4 designated by the address information.
Though the foregoing is described with regard to one channel, an arrangement can be adopted in which a plurality of channels having different numbers of cycles of the A- and B-phase signals are provided. A predetermined number of items of angle information are interpolated in one wavelength of each channel, and the absolute position of a movable element is detected by using the interpolated data on each channel. An absolute position encoder adapted to detect the angle information by using a plurality of channels having different numbers of cycles has already been filed for patent as International Serial No. PCT/JP89/00506.
If the resolution of the AD converters is, say, eight bits, then, when an A-phase signal K.multidot.sin.theta. and a B-phase signal K.multidot.sin.theta. are subjected to an AD conversion, a resolution in which the result of calculation with regard to the angle .theta. is 10 bits can be expected.
However, the ROM used in the conventional encoder is one in which the output is composed of eight bits; hence, angle data composed of more than eight bits cannot be delivered as an output. If a ROM having an output of 16 bits is used, the expected angle data can be outputted and highly precise position detection is possible. However, the configuration in such case is larger in size and has a greater number of signal lines and therefore cannot be employed in an absolute position encoder which requires a reduced size.